Friction stir welding is a method in which, by rotating a round pin (called “a rotary tool”) which has been inserted into a welding joint between extruded frame members and moving the rotary tool along a welding line of the welding joint, the welding portion is heated, softened and plastically fluidized and is solidly welded. The rotary tool is comprised of a small diameter portion which is inserted into the welding joint and a large diameter portion which is positioned outside of the small diameter portion of the rotary tool. The small diameter portion and the large diameter portion of the rotary tool have the same axis. A boundary between the small diameter portion and the large diameter portion of the rotary tool is inserted to a small extent into the welding joint during welding.
The above-stated technique is disclosed, for example in WO95026254 (TWI).
Welding of large frame members maintaining close tolerances is known to be difficult due to the dimensional variations.
EP893190 discloses the use of sloped surfaces to compensate for dimensional variations causing a gap between the frames to be joined.
In EP0947280 an overlap joint is used to supply extra metal to the joint at friction stir welding of components having a gap at the joint area. There is no discussion of compensation for dimensional variations by adjustment of the frame structure.
EP1057574 discloses the use of an intermediate member overlapping the frame members to be joined at the joint area. No reason for the overlap is given, nor is the use of an integrated extension from one of the frame members considered.